Make  ComputeTestCase.test_state_revert faster

Reuse instance instead of creating a new one every time.

Before:
nova.tests.compute.test_compute.ComputeTestCase.test_state_revert 11.032 seconds
After:
nova.tests.compute.test_compute.ComputeTestCase.test_state_revert 6.415 seconds

Change-Id: I67a5a971c3898d4fffed9e6b4640c6c0f3c62a56
Merge "Make  ComputeTestCase.test_state_revert faster"